OFDM
In a wireless communication network, such as the 3rd generation (3G) wireless cellular communication standard and the 3GPP long term evolution (LTE) standard, it is desired to concurrently support multiple services and multiple data rates for multiple users in a fixed bandwidth channel. One scheme adaptively modulates and codes symbols before transmission based on current channel estimates. Another option available in LTE, which uses orthogonal frequency division multiplexed access (OFDMA), is to exploit multi-user frequency diversity by assigning different sub-carriers or groups of sub-carriers to different users or UEs (user equipment). The system bandwidth can vary, for example, from 1.25 MHz to 20 MHz. The system bandwidth is partitioned into a number of subcarriers, e.g., 1024 subcarriers for a 5 MHz bandwidth.
The following standardization documents are incorporated herein by reference: 36.211, 3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Physical Channels and Modulation (Release 8), v 1.0.0 (2007-03); R1-01057, “Adaptive antenna switching for radio resource allocation in the EUTRA uplink,” Mitsubishi Electric/Nortel/NTT DoCoMo, 3GPP RAN1#48, St. Louis, USA; R1-071119, “A new DM-RS transmission scheme for antenna selection in E-UTRA uplink,” LGE, 3GPP RAN1#48, St. Louis, USA; and “Comparison of closed-loop antenna selection with open-loop transmit diversity (antenna switching within a transmit time interval (TTI)),” Mitsubishi Electric, 3GPP RAN1#47bis, Sorrento, Italy. According to the 3GPP standard, the base station is enhanced, and is called the “Evolved NodeB” (eNodeB).
MIMO
In order to further increase the capacity of a wireless communication system in fading channel environments, multiple-input-multiple-output (MIMO) antenna technology can be used to increase the capacity of the system without an increase in bandwidth. Because the channels for different antennas can be quite different, MIMO increases robustness to fading and also enables multiple data streams to be transmitted concurrently.
While MIMO systems perform well, they also can increase the hardware and signal processing complexity, power consumption, and component size in transceivers. This is due in part to the fact that each receive antenna requires a receive radio frequency (RF) chain, which typically comprises a low noise amplifier, a frequency down-converter, and an analog to digital converter. Similarly, each transmit antenna element requires an RF chain that comprises a digital to analog converter, a frequency up-converter, and a power amplifier.
Moreover, processing the signals received in spatial multiplexing schemes or with space-time trellis codes requires receivers where the complexity can increase exponentially as a function of the number of antenna.
Antenna Selection
Antennas are relatively simple and cheap, while RF chains are considerably more complex and expensive. Antenna selection reduces some of the complexity drawbacks associated with MIMO systems. Antenna selection reduces the hardware complexity of transmitters and receivers by using fewer RF chains than the number of antennas.
In antenna selection, a subset of the set of available antennas is adaptively selected by a switch, and only signals for the selected subset of antennas are connected to the available RF chains for signal processing, which can be either transmitting or receiving. As used herein, the selected subset, in all cases, means one or more of all the available antennas in the set of antennas. Note, that invention also allows multiple subsets to be used for training. For example, there can be four antennas and one RF chain, or eight antennas and two RF chains, which includes four subsets.
Antenna Selection Signals
Pilot Tones or Reference Signals
In order to select the optimal subset of antennas, all channels corresponding to all possible transmit and receive antenna subsets need to be estimated, even though only a selected optimal subset of the antennas is eventually used for transmission.
This can be achieved by transmitting antenna selections signals, e.g., pilot tones, also called reference signals, from different antennas or antenna subsets. The different antenna subsets can transmit either the same pilot tones or use different ones. Let Nt denote the number of transmit antennas, Nr the number of receive antennas, and let Rt=Nt/Lt, and Rr=Nr/Lr be integers. Then, the available transmit (receive) antenna elements can be partitioned into Rt(Rr) disjoint subsets. The pilot repetition approach repeats, for Rt×Rr times, a training sequence that is suitable for an Lt×Lr MIMO system. During each repetition of the training sequence, the transmit RF chains are connected to different subsets of antennas. Thus, at the end of the Rt×Rr repetitions, the receiver has a complete estimate of all the channels from the various transmit antennas to the various receive antennas.
In case of transmit antenna selection in frequency division duplex (FDD) systems, in which the forward and reverse links (channels) are not identical, the receiver feeds back the optimal set of the selected subset of antennas to the transmitter. In reciprocal time division duplex (TDD) systems, the transmitter can perform the selection independently.
For indoor LAN applications with slowly varying channels, antenna selection can be performed using a media access (MAC) layer protocol, see IEEE 802.11n wireless LAN draft specification, I. P802.11n/D1.0, “Draft amendment to Wireless LAN media access control (MAC) and physical layer (PHY) specifications: Enhancements for higher throughput,” Tech. Rep., March 2006.
Instead of extending the physical (PHY) layer preamble to include the extra training fields (repetitions) for the additional antenna elements, antenna selection training is done by the MAC layer by issuing commands to the physical layer to transmit and receive packets by different antenna subsets. The training information, which is a single standard training sequence for a Lt×Lr MIMO system, is embedded in the MAC header field.
OFDMA Structure in LTE
The basic uplink transmission scheme is described in 3GPP TR 25.814, v1.2.2 “Physical Layer Aspects for Evolved UTRA.” The scheme is a single-carrier transmission (SC-OFDMA) with cyclic prefix (CP) to achieve uplink inter-user orthogonality and to enable efficient frequency-domain equalization at the receiver side.
LTE Reference Signals
3GPP LTE envisages using two kinds of reference signals. Both the reference signals are transmitted in one or more of the long blocks (LB) of the TTI, or its short blocks, if available.
Data Modulation Reference Signals
The data modulation (DM) reference signal is transmitted along with data in the subcarriers assigned to the user equipment. These signals help the eNodeB (Base station) receiver to acquire an accurate estimate of the channel, and thereby coherently decode the received signal.
Broadband Sounding Reference Signals (SRS)
The broadband SRS is meant to help the eNodeB to estimate the entire frequency domain response of the uplink channel from the user to the eNodeB. This helps frequency-domain scheduling, in which a subcarrier is assigned, in principle, to the user with the best uplink channel gain for that subcarrier. Therefore, the broadband SRS can occupy the entire system bandwidth, e.g., 5 MHz or 10 MHz. Alternatives have also been proposed in which the broadband SRS occupies a fraction of the system bandwidth and is frequency hopped over multiple transmissions in order to cover the entire system bandwidth.